Numerical simulation of three-dimensional(3-D)unsteady Euler equations was conducted to investigate the unsteady flows around co-axial rotors in hover.The thrust performances of two rotors became worse because of the interaction between them

and the influence of the upper rotor on the lower rotor was greater than that of the lower rotor on the upper rotor.The downwash velocity at the lower rotor plane was much larger than that at an isolated rotor plane

and the downwash velocity at the upper rotor plane was a little larger than that at an isolated rotor plane.The downwash velocity and thrust coefficient became larger when the collective angle of blades was added.With the increase of distance between the two co-axial rotors

the thrust coefficient of the upper rotor increased

and the total thrust coefficient reduced

because the thrust coefficient of the lower rotor dropped greater than the increment of the upper rotor.